> Consider code like:
>
> public class MainClass {
> public MainClass() {
> System.out.println("Constructor called!");
> }
> public static void main() {
> System.out.println("main called!");
> }
> }
>
> and compile and run it, with preview enabled, like:
>
> $ javac /tmp/MainClass.java
> $ java --enable-preview -classpath /tmp MainClass
> Constructor called!
> main called!
>
>
> That is wrong, as the `main` method is static, and there is no need to create
> a new instance of the class.
>
> The reason is that as launcher attempts to invoke the main method, it goes in
> the following order: 1) static-main-with-args; 2) instance-main-with-args; 3)
> static-main-without-args; 4) instance-main-without-args. But, for the
> instance variants, the code first creates a new instance of the given class,
> and only then attempts to lookup the `main` method, and will pass to the next
> option when the `main` method lookup fails. So, when invoking
> static-main-without-args, the current class instance may be created for
> instance-main-with-args, which will then fail due to the missing
> `main(String[])` method.
>
> The proposed solution to this problem is to simply first do a lookup for the
> `main` method (skipping to the next variant when the given main method does
> not exist, without instantiating the current class).
>
> There is also a relatively closely related problem: what happens when the
> constructor throws an exception?
>
> public class MainClass {
> public MainClass() {
> if (true) throw new RuntimeException();
> }
> public void main() {
> System.out.println("main called!");
> }
> }
>
>
> when compiled an run, this produces no output whatsoever:
>
> $ javac /tmp/MainClass.java
> $ java --enable-preview -classpath /tmp MainClass
> $
>
>
> This is because any exceptions thrown from the constructor are effectively
> ignored, and the launcher will continue with the next variant. This seems
> wrong - the exception should be printed for the user, like:
>
> $ java --enable-preview -classpath /tmp/ MainClass
> Exception in thread "main" java.lang.RuntimeException
> at MainClass.<init>(MainClass.java:3)
>
>
> This patch proposes to do that by not consuming the exceptions thrown from
> the constructor, and stop the propagation to the next variant.
Jan Lahoda has updated the pull request incrementally with three additional
commits since the last revision:
- Reflecting code formatting suggestion.
- First lookup the main method, and only then the constructor.
- Attempting to solve JDK-8329581 by only ignoring j.l.NoSuchMethodError
-------------
Changes:
- all: https://git.openjdk.org/jdk/pull/18753/files
- new: https://git.openjdk.org/jdk/pull/18753/files/2022aa5a..c007f61e
Webrevs:
- full: https://webrevs.openjdk.org/?repo=jdk&pr=18753&range=02
- incr: https://webrevs.openjdk.org/?repo=jdk&pr=18753&range=01-02
Stats: 117 lines in 2 files changed: 73 ins; 5 del; 39 mod
Patch: https://git.openjdk.org/jdk/pull/18753.diff
Fetch: git fetch https://git.openjdk.org/jdk.git pull/18753/head:pull/18753
PR: https://git.openjdk.org/jdk/pull/18753
